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Sense of touch turns bats into acrobats

Ask any elementary school student how bats get around, and they are likely to give you the well-known answer: echolocation. But a new study suggests that the furry fliers use more than just these high-frequency sound blips to get from point A to point B. They also have a keen sense of touch that allows them to detect and respond to the motion of air over their wings, turning them into agile aerial acrobats.

Bats’ thin, flexible wings—a thumb and four fingers connected by webbing—stretch and reshape during flight, unlike those of birds and insects. Their agility in the air demands quick, precise wing movements and a constant adjustment of tiny muscles in the wing membrane. They also use their wings for other delicate tasks, like holding food and cradling young. To adjust their complex wings for the job at hand, they must integrate a variety of sensory feedback.

To understand how bats feel their way around the sky, researchers mapped the arrangement of touch-sensitive receptors on the wings of big brown bats (Eptesicus fuscus). They found high concentrations of sensors known as Merkel cells at the base of tiny hairs that cover the wings, they report online today in Cell Reports. Merkel cells are highly sensitive to fine touch, and when airflow jostles a hair, the cells react to its motion. Because the wing is hairy on both sides, bats are sensitive to airflow both above and below, says neuroscientist Cynthia Moss of Johns Hopkins University in Baltimore, Maryland, a senior author of the study.

Bats rely on finely tuned patterns of airflow to keep themselves aloft, including vortices at the leading edges of the wings that enhance lift. The animals may use their hairs to sense these vortices and make midair corrections, says animal flight researcher Anders Hedenström of Lund University in Sweden, who was not involved with the research. "The problem for the bat is … their aerodynamics is very hard to control in slow flight, because these [vortices] are very unsteady phenomena,” he says. “They need some kind of information about the flow near the wing."

The team also studied how bats' brains responded to the stimulation of their wings. They measured the response of neurons in the bat's somatosensory cortex—the area of the brain that responds to touch—when they blew small bursts of air on the wing or lightly touched it. The bats' neurons fired quickly in response but quieted down soon after, indicating that air currents could produce rapid but brief feedback, suitable for making swift adjustments in flight.

The researchers also found an unexpected arrangement in the bats' neural circuitry, which they uncovered by tracing neurons in the wings back to the spinal column. In most mammals, sensory neurons in the hand or forelimb are connected to upper parts of the spinal column. But in parts of the bats' wings, the neurons' connections were mixed, with some connected to the upper spinal column and some connected to a lower part. This, the team says, can be explained by the embryonic development of the bat's wing, which originates from both the forelimb and the torso.

Most prior research in bat movement has focused on visual sensing or echolocation, and a better understanding of how bats use their sense of touch could have practical applications, Moss says. It could be applied to designing new types of wind-sensing aircraft with flexible wings that could be more maneuverable than modern aircraft.

"I think this is exciting and that it takes us one step forward in the understanding of bat flight, which is a very complex phenomenon," Hedenström says. "We still need to research exactly what information these nerves convey to the bat and how it’s processed," he says, but "there is a need for the bat to get this information. I think that’s pretty clear."